Stud welding is widely used in the manufacture of such items as cookware, heavy machinery and furniture. Studs are available in a variety of shapes and sizes, ranging from one quarter inch long with a threaded end to one foot long, smooth, and hollow. Most studs are cylindrical to simplify handling and each stud is welded by its tip to a parent metal with an electric welding process which uses the stud itself as an electrode.
Since the advent of the stud welding gun in the late 1950's, the use of the headed shear studs in building and bridge deck construction has become widespread. Typically 10,000 to 200,000 studs are used at a single location. The type of stud is shown in FIG. 2A. It is a bolt-like piece of metal, usually about five inches long and 3/4 inch in diameter, with a head that is about 11/4 inch in diameter. Studs are welded through preformed corrugated steel decking to underlying steel substructures, typically I-beams, with ferrules, small ceramic rings, acting as weld molds. A ferrule is shown in FIG. 2A. Studs are welded in rows on structural steel decking then concrete is then poured over the studs and the decking, effectively creating a composite structure. FIG. 2B shows a sectional view of the components in a floor of this type. The studs act to prevent the concrete from moving relative to the steel as a result of thermal or mechanical stresses. The concrete works in compression and the steel in tension.
The welding of shear studs in the construction industry is typically done using a hand-held semi-automatic "stud gun" as shown in FIG. 2C. To weld a stud, an operator with a stud gun picks up a stud which has been placed next to the weld site. The operator then loads the stud into the stud chuck, with the tip pointing away from the gun along its centerline. Aligning the protruding stud tip with a ferrule which has been placed directly over the weld site, the operator then threads the stud through the ferrule until both the stud is touching the steel decking, and the end of the stud gun has engaged the ferrule. At this point, the stud and ferrule are ready for welding. Because the gun is resting on the ferrule, its height remains constant through the welding cycle even though the stud, once it is being welded, provides neither support nor reference for the gun. To initiate the weld, the operator pulls a trigger on the gun which closes a circuit in the weld gun controller. When the circuit is closed, current flows through the stud and the stud is lifted by a solenoid in the gun to a prescribed height of about one quarter of an inch, thus creating an arc. The arc, carrying between 1,600 and 2,400 amps melts both the base metal and the tip of the stud. After about one half to three quarters of a second, the welding current in halted and solenoid is deenergized, thus plunging the stud in a pool of molten metal which very rapidly solidifies. The ferrule helps mold the metal around the stud, forming a strong weld. Almost immediately after the stud is plunged back into the base metal, the operator is free to pull the gun away from it and continue on to the next weld site. The actual welding process takes about one second. Typically two workers are involved. One will lay out studs and ferrules where the studs are to be welded and the other will follow with a stud gun and weld them into place. The first worker, in turn, breaks the ferrules away from the studs. A significant amount of time is spent moving equipment and materials from one locale to the next and performing random testing to insure good weld quality. This testing usually means beating sideways (horizontally) on the head of a welded stud with a sledge hammer to see if the stud will or will not break off at the weld.
A great number of automatic stud welding systems are currently in operation in factory environments, welding a vast array of studs. However, none of the systems deal with studs which are headed and as large as the ones used in buildings and bridges. There are several problems associated with feeding large, headed studs into a welding chuck. Vibratory/pneumatic feed systems used in most automatic stud welders do not lend themselves to the inertia and irregular shape of a shear stud. The bulk and weight of such a feed system would be unacceptable in a portable stud welding system. There are no known solutions to the problem of automatic feeding of both studs and ferrules and providing proper positioning for both prior to welding.
There have been earlier attempts at speeding, if not automating, the shear stud welding process. Roughly 20 years ago, the Nelson Stud Welding Company, now a Division of TRW, marketed a device which held 4 stud guns and rolled along an I-beam. The operator manually indexed, loaded and welded the studs. While the machine made accurate, high quality welds, it was slower than a single operator with a single stud gun. The Nelson welding machine used the edge of the I-beam or deck as a guide and was moved along manually by the operator. However, problems arise when there are obstructions such as rebar and concrete forms in place before the stud welding operation commences.
Building decks represent more difficulties in terms of guidance and obstacles. The studs must be welded through decking into underlying beams which often abut vertical structural members. Vertical structural members in a building present periodic obstacles in what otherwise may be a simple, clear path for a stud welder. Corrugations in the decking present further difficulties because the studs are placed and welded in the valleys, while the machine itself must clear the peaks. Another problem is that there may also be a thick layer of a rust inhibitive coating which must be ground away before welding can take place.